Index: lib/fuzzer/FuzzerLoop.cpp
===================================================================
--- lib/fuzzer/FuzzerLoop.cpp
+++ lib/fuzzer/FuzzerLoop.cpp
@@ -658,7 +658,9 @@
         Size <= CurrentMaxMutationLen)
       NewSize = MD.MutateWithMask(CurrentUnitData, Size, Size,
                                   II.DataFlowTraceForFocusFunction);
-    else
+    
+    // If MutateWithMask either failed or wasn't called, call default Mutate.
+    if (!NewSize)
       NewSize = MD.Mutate(CurrentUnitData, Size, CurrentMaxMutationLen);
     assert(NewSize > 0 && "Mutator returned empty unit");
     assert(NewSize <= CurrentMaxMutationLen && "Mutator return oversized unit");
Index: lib/fuzzer/FuzzerMutate.cpp
===================================================================
--- lib/fuzzer/FuzzerMutate.cpp
+++ lib/fuzzer/FuzzerMutate.cpp
@@ -542,6 +542,7 @@
     if (Mask[I])
       T[OneBits++] = Data[I];
 
+  if (!OneBits) return 0;
   assert(!T.empty());
   size_t NewSize = Mutate(T.data(), OneBits, OneBits);
   assert(NewSize <= OneBits);